Many types of wheels employ metal spokes to form a connection structure between the hub (which forms the center of the wheel and is usually attached to an axel) and the rim (which forms the outer circumference of the wheel and usually has a rubber tire affixed thereto). A spoked wheel is used commonly in bicycles because it allows the wheels to be lighter than if a solid structure was used to connect the hub and rim. Since extra weight increases the inertia of a wheel and therefore slows the ability of the rider to accelerate quickly or climb hills, many attempts have been made in the past to optimize the materials and construction of bicycle wheels.
Bicycle spokes are typically constructed of stainless steel due to its high strength, good fatigue life, excellent corrosion resistance, and low cost. In a common road bicycle, stainless steel spokes typically weigh about 8 grams each and there may be a total of up to 64 spokes split between the two wheels. To reduce weight, the spokes can be manufactured with a smaller diameter in the middle of the spoke, a process called butting. Butted spokes can be in the 4 to 6 gram range, however they cannot be reduced below a certain cross sectional area because the tensile strength and fatigue properties become insufficient to warrant extended use without failure. Even with these reduced weights, bicycle spokes still make up a significant portion of the weight of the wheels. In the past, spoke manufacturers have utilized materials such as titanium, fiber-reinforced plastic, and other polymers to try to decrease the weight further, but none of these options has attained mainstream success.
In past implementations, each of the materials discussed above have had shortcomings when compared to stainless steel: titanium spokes have low stiffness, higher costs, and reduced fatigue life compared to steel. Fiber reinforced plastic (or carbon fiber) can fail catastrophically if one or more spokes fail individually. Novel plastics, such as polybenzoxazole require the use of custom wheel components and are therefore not desirable. In addition, novel plastics introduced in the past have required a protective jacket around the core to protect the plastic from sources of degradation such as UV exposure, abrasion by foreign objects, and moisture. Because of these disadvantages, stainless steel spokes remain the prevailing material used in bikes today.
Polymer technology has greatly advanced however in the past decades to the point where materials with sufficiently favorable properties have been commercialized. These properties include strength to weight ratio, creep resistance, UV resistance, moisture resistance, and cost to manufacture. Because of this, thick protective coatings are no longer required in some cases. Also, creep resistant polymers have been developed increasing the number of materials which could theoretically be used to make bicycle spokes.
The technology necessary for effective utilization of high-performance materials in typical bike wheel configurations has not caught up with the materials themselves. Prior art does not teach a technique which allows for integration with common hubs and rim connections. In fact, prior art describes connections and spoke implementations which preclude integration with standard wheel components. This is a critical omission because the industry is largely standardized. Accordingly, products with custom requirements are unlikely to be adopted and commercially successful.
The idea of specifically utilizing a non-rigid material to replace stainless steel was proposed as early as 1990 in U.S. Pat. No. 5,110,190 by Johnson. In this patent, generic connection methods for fabricating a wheel from non-rigid spokes are disclosed, but the ability to construct the spoke such that it can be incorporated into a standard wheel is not taught. In fact, many of the materials and connections modalities discussed are explicitly not compatible with traditional wheel components and require the use of a specially designed system. For example, creating an enlarged head through affixation of a bulb or knot tying means the enlarged end will, by definition, be too large to insert through the hub holes. The opposite end of the spoke taught by Johnson uses a non-standard rim-spoke connection, incompatible with traditional nipples that those familiar with the bicycle industry use. Other methods presented suffer from this fundamental flaw—incompatibility with standard hubs, rims, and nipples.
Another flexible spoke design was proposed by Campbell in U.S. Pat. No. 6,036,281 which utilized liquid crystal fibers and an extruded jacket. The jacket is necessary to protect the fibers which are susceptible to breakage when loaded transverse to their main axis of orientation. This combination produces a spoke that is over 3 mm in diameter which is too large to fit through a standard hub hole. Additionally, the rim connection taught in the patent is also incompatible with a standard rim hole or nipple because the fiber must go through the center of the threaded section. Non-standard hubs and rims are required to construct a wheel out of these spokes.
Lubecki, in U.S. Pat. Nos. 7,988,240, 8,313,154, and 8,794,714 discloses yet another spoke connection strategy in which a custom hub with cradles cut into the hub flanges is used to interlock a flexible spoke. While this strategy helps to get around earlier assembly problems, it still requires a custom hub to be manufactured and thus does not address the fundamental problem.
Even rigid alternate materials have struggled with compatibility with standard equipment. The fiber-reinforced-plastic spoke proposed by Imao in U.S. Pat. No. 4,729,605 adds several incremental components between the hub connection and the rim connection which add a significant amount of weight to the spoke. Introduction of extra components quickly eliminates the weight advantages of non-steel materials.
In light of the past aforementioned attempts to create an improved wheel connection system for flexible spokes, it is clear that none have been able to incorporate the spokes with standard equipment.